Production of semiconductor integrated circuits and other micro-scale devices typically requires formation of multiple metal layers on a wafer or other substrate. By electroplating metals layers in combination with other steps, such as planarizing, etching and photolithography, patterned metal layers forming the micro-scale devices are created.
Electroplating is performed with the substrate, or one side of the substrate, in a bath of liquid electrolyte, and with electrical contacts touching a conductive layer on the substrate surface. Electrical current is passed through the electrolyte and the conductive layer. Metal ions in the electrolyte deposit or plate out onto the substrate, creating a metal layer on the substrate. The metal ions also tend to plate out onto the electrical contacts as well. This affect, referred to as “plate-up” changes the electric field around the contacts, causing non-uniform plating. The metal plated onto the electrical contacts consequently must be removed, adding to the time requirements and complexity of the manufacturing process.
So called dry or closed contact rings have been developed to avoid plate-up of the contacts. In these designs, a seal ring seals the electrolyte away from the electrical contacts. The electrical contacts touch the conductive layer on the substrate at the perimeter of the substrate. The seal ring contacts the substrate surface radially inwardly of the electrical contacts, so that the contacts remain isolated from the electrolyte.
Although the use of a seal in a dry contact ring solves the plate-up problem, dry contact rings have their own disadvantages. Initially, the seal of a dry contact ring necessarily contacts or covers an annular shaped area on the substrate surface, which area cannot be used to form devices. Hence, a fraction of the useable substrate surface must be sacrificed if a seal is used. The seal must also not unduly disturb the electric field around the edge of the wafer, or electroplating quality will be degraded. In some processors, the seal can also plate up (i.e., metal gets plated onto the seal) over successive wafer plating cycles. Avoiding seal plate up is also significant in providing uniform high quality metal plated wafers. The seal must also perform reliably and consistently over a large number of plating cycles, without leaking and with minimal sticking to the wafer after the plating cycle.